#include <wchar.h>

#define MAX(a,b) ((a)>(b)?(a):(b))
#define MIN(a,b) ((a)<(b)?(a):(b))

static wchar_t *twoway_wcsstr(const wchar_t *h, const wchar_t *n)
{
    const wchar_t *z;
    size_t l, ip, jp, k, p, ms, p0, mem, mem0;

    /* Computing length of needle */
    for (l=0; n[l] && h[l]; l++);
    if (n[l]) return 0; /* hit the end of h */

    /* Compute maximal suffix */
    ip = -1; jp = 0; k = p = 1;
    while (jp+k<l) {
        if (n[ip+k] == n[jp+k]) {
            if (k == p) {
                jp += p;
                k = 1;
            } else k++;
        } else if (n[ip+k] > n[jp+k]) {
            jp += k;
            k = 1;
            p = jp - ip;
        } else {
            ip = jp++;
            k = p = 1;
        }
    }
    ms = ip;
    p0 = p;

    /* And with the opposite comparison */
    ip = -1; jp = 0; k = p = 1;
    while (jp+k<l) {
        if (n[ip+k] == n[jp+k]) {
            if (k == p) {
                jp += p;
                k = 1;
            } else k++;
        } else if (n[ip+k] < n[jp+k]) {
            jp += k;
            k = 1;
            p = jp - ip;
        } else {
            ip = jp++;
            k = p = 1;
        }
    }
    if (ip+1 > ms+1) ms = ip;
    else p = p0;

    /* Periodic needle? */
    if (wmemcmp(n, n+p, ms+1)) {
        mem0 = 0;
        p = MAX(ms, l-ms-1) + 1;
    } else mem0 = l-p;
    mem = 0;

    /* Initialize incremental end-of-haystack pointer */
    z = h;

    /* Search loop */
    for (;;) {
        /* Update incremental end-of-haystack pointer */
        if (z-h < l) {
            /* Fast estimate for MIN(l,63) */
            size_t grow = l | 63;
            const wchar_t *z2 = wmemchr(z, 0, grow);
            if (z2) {
                z = z2;
                if (z-h < l) return 0;
            } else z += grow;
        }

        /* Compare right half */
        for (k=MAX(ms+1,mem); n[k] && n[k] == h[k]; k++);
        if (n[k]) {
            h += k-ms;
            mem = 0;
            continue;
        }
        /* Compare left half */
        for (k=ms+1; k>mem && n[k-1] == h[k-1]; k--);
        if (k <= mem) return (wchar_t *)h;
        h += p;
        mem = mem0;
    }
}

wchar_t *wcsstr(const wchar_t *restrict h, const wchar_t *restrict n)
{
    /* Return immediately on empty needle or haystack */
    if (!n[0]) return (wchar_t *)h;
    if (!h[0]) return 0;

    /* Use faster algorithms for short needles */
    h = wcschr(h, *n);
    if (!h || !n[1]) return (wchar_t *)h;
    if (!h[1]) return 0;

    return twoway_wcsstr(h, n);
}
